High current thermionic hollow cathode lamp

ABSTRACT

A thermionically emitting hollow cathode for very high currents up to 400 amperes AC particularly suitable for xenon-filled wallstabilized lamps. It comprises a hollow cylindrical body of porous tungsten, open in the direction of the arc and having a coil of tungsten wire lining the cavity walls except for a region at the rear deep within the cavity. The cavity wall but not the face nor the outside of the electrode are impregnated with emission material, suitably barium thorate. The cooler shank end of the cavity in which the electrode coil does not extend serves as a dead space into which gas can expand during the AC cycle, thereby reducing the rate of gas flow in and out of the open end.   D R A W I N G

i United States Patent [72] Inventor John E. White Cleveland Heights,Ohio [21 Appl. No. 769,038 [22] Filed Oct. 21,1968 [45] Patented Jan.26, 1971 General Electric Company [73] Assignee a corporation of NewYork [54] HIGH CURRENT THERMIONIC HOLLOW 3,014,156 12/1961 Osterhammelet al 313/218X 3,029,359 4/1962 White 313/211X 3,076,916 2/1963 Koppius313/346 3,244,929 4/1966 Kuhl 313/346X Primary Examiner-John KominskiAssistant ExaminerPalmer C. Demeo Attorneys-Ernest W. Legree, Henry P.Truesdell, F rank L. Neuhauser, Oscar B. Waddell and Melvin M.Goldenberg ABSTRACT: A thermionically emitting hollow cathode for veryhigh currents up to 400 amperes AC particularly suitable forxenon-filled wall-stabilized lamps. It comprises a hollow cylindricalbody of porous tungsten, open in the direction of the arc and having acoil of tungsten wire lining the cavity walls except for a region at therear deep within the cavity. The cavity wall but not the face nor theoutside of the electrode are impregnated with emission material,suitably barium [56] References cued thorate. The cooler shank end ofthe cavity into which the elec- UNITED STATES PATENTS trode coil doesnot extend serves as a dead space into which 2,104,652 1/1938 lnman313/184X gas can expand during the AC cycle, thereby reducing the rate2,878,409 3/1959 Levi 313/346 of gas flow in and out of the open end.

r i i Z l8 l9 3 5 9 l0 i I l4 1 J k PATENTEU JAMES |97| ..e m DH M m h1M0 J His A t trorneg HIGH CURRENT THERMIONIC HOLLOW CATIIODE LAMPBACKGROUND OF THE INVENTION The invention relates to therrnionicallyemitting high current electrodes for use in very high power lamps,particularly xenon-filled wall-stabilized lamps. In my U. S. Pat. No.3,029,359 I have described and claimed a thermionic hollow cathode whichoperates well with a low cathode potential drop (l.2 volts) and withlittle envelope blackening at currents up to 75 amperes r.m.s. Thatcathode comprises a hollow cuplike body of tungsten open towards thefront. that is in the direction of the are, and has a tungsten coillining the cavity walls. Emission material consisting of a bariumcontaining compound-is coatedon the coil and lodged in the intersticesbetween the turns of the coil and the cavity wall.

The object of my invention is to provide a yet more efficient cathodeand one capable of operation at much higher currents, at least up to 400amperes r.m.s.

SUMMARY OF THE INVENTION A thermionic-emitting high current cathodeaccording to my invention comprises a hollow body of porous tungsten,suitably a hollow cylinder open towards the front, that is in thedirection of the are. A coil of refractory metal wire, suitablytungsten, lines the cavity wall except for an inactive region or deadspace at the rear, that is in the deepest portion of the cavity.Emission material, suitably barium thorate, is embedded in the pores ofthe tungsten of the inner surface of the electrode cavity. The materialis filled into the pores to the depth required to hold the quantitynecessary for the intended life of the lamp. The impregnated emissionmaterial causes the inner surface of the electrode. cavity to show awhitish haze. The porous structure of the tungsten mechanically protectsthis reservoir of emission mix from peeling and also from disintegrationby exposure to the are. The structure allows the low-work functionemission material to be transferred or dispensed to the coil in acontrolled manner.

The application of emission material to the face and to the outercylindrical surface of the electrode is carefully avoided in order toprevent undesirable burning of the are in a spotmode anywhere and toconfine it to the cavity where it burns in a diffused mode.

A factor causing erosion of tungsten and envelope darkening in hollowelectrodes is breathing "during AC operation. Within the electrodecavity where the power density is extremely high, essentially all metalvapor, and particularly that of the easily ionized emission mixcomponent, is ionized during burning of the are. The ions can move onlya millimeter or so during a half-cycle of the AC voltage. During thecathode half-cycle, the field tends to retain the ions within thecavity; during the anode half-cyele, the field may tend to force theions out (although a negative anode field within the cavity is notexcluded). The overall restraining effect of the cavity can beoverpowered however by the heating and cooling of the gas during the ACcycle. The motion of an ion during a half-cycle under the action ofexpanding gas is likely to be much greater than under the action of thefield. Various experiments have shown this breathing" effect capable oferoding hot tungsten. The breathing effect carries metal vaporsincluding tungsten out of the electrode cavity and into the lamp volumeproper and deposition of the metal vapors on the envelope walls causeblackening.

My invention greatly reduces or substantially eliminates envelopedarkening due to breathing by reason of two features:

l. The front end of the electrodes is much hotter than the rear or shankend, for instance l690 C. vs. 1040 C.

2. The cathode emitter coil does not extend the entire depth of thecathode cavity.

As a result of the above factors, the are burns towards the frontportion of the cavity, As the arc current builds up during the AC cycle,heated gas tends to blow into and out of the inactive or dead space atthe rear into which the arc does not ex tend. However the gas in thepart of the lamp envelope where the positive column occurs is heating atthe same time as the gas in the front portion of the cavity and abalancing effect takes place. The result is that there is much less gastransfer back and forth through the electrode entrance than there isbetween back and front portions of the cavity. To a first approximation,if the ratio of live space-- to dead space" within the electrode is thesame as the ratio of live space to dead spacein the rest of the lamp,there will be no motion of gas through the cavity mouth during the ACcycle. and blackening from this cause will be minimized. Accordingly. ina preferred embodiment of my invention. this ratio is observed asbetween electrode and lamp volumes.

In order to assure'the proper rate of dispensing of emission mix fromthe reservoir to the coil within cathode, an emission materialappropriate to the operating temperature of the cathode must be used. Apreferred emission material is barium thorate BaTh0- Other materialswhich may be used as electron emitters are barium zirconate, bariumaluminate, strontium thorate and thol'ia. In known manner, reducingagents may be added to the emission mix which are capable of reacting atthe rate necessary to compensate for the evaporation of barium takingplace. An advantage of using BaThO- is that as it decomposes, straightevaporation of Ba0 takes place and no reduction is necessary.

DESCRIPTION OF DRAWINGS FIG. 1 is a side view of a high powered xenonlamp provided with cathodes embodying the invention. A central sectionof the lamp has been cut out in order to shorten the FIG. and one end ofthe lamp and its electrode have been sectioned.

FIG. 2 is a graphshowing the volt ampere characteristic of the lamp.

DESCRIPTION OF PREFERRED EMBODIMENT Referring to FIG. 1 of the drawing,illustrated lamp assembly 1 comprises a tubular envelope 2 made ofquartz and containing xenon as the ionizable filling gas. At the ends ofthe envelope are mounted the thermionic electrodes 3 supported oninternal rod conductors 4 which extend through reduced tubular vitreousextensions or necks 5. The necks 5 are made up of several vitreoussections having intermediate coefficients of expansion, the .last beingsuitable for scaling to the thin thimblelike edge 6 of tubular externalterminal member 7 in which rod 4 is engaged. The hermetic seal occursbetween the neck 5 and the thin edge 6 of terminal member 7; elsewherethere must not be any bond between the vitreous and metal parts and thefine wire coil 8 wrapped around the rod next to the electrode assuresclearance between the rod and neck at that point. The lamp iswater-cooled by means of a glass jacket 9 which surrounds the envelope2. A water-seal is made at each end by means of discs 10 and I1 drawntogether by screws 12; one disc is flat and the other is chamferedinternally and externally to accommodate rubber sealing rings I3, 14.The water inlet and outlet are indicated at l5, l6 respectively and aspiral glass rod 17 in the space between the envelope 2 and the jacket 9assures sufficient velocity of water flow to prevent boiling at anyplace.

The electrodes 3 prop er each comprise a generally cylindrical hollowbody 18 of porous tungsten open towards the front, that is in thedirection of the arc. Suitably the porous tungsten of the electrode bodymay be percent of theoretical density. Within the body, a close woundemitter coil 19 of tungsten wire lines the cylindrical walls of thecavity from the front or open end to about three-fourths of the cavitydepth. The por tion of the cavity in which the wall is lined by the coilmay be referred to as the live space within the electrode; the unlinedportion'at greater depth may be referred to as dead space."

is mechanically protected from peeling. The emission mix is alsoprotected from disintegration by exposure to the arc by reason of itsdispersion through the porous tungsten structure. Transfer or dispensingof the low-work function material from the porous wall to the coiloccurs in a controlled manner throughout the life of the lamp.

One way of embedding the emission material powder in the pores of thetungsten is to suspend it in a hydrophobic solvent such as toluene orn-heptane, and flow it onto the tungsten surface. Another way is tomount the electrode in a fixture, entrain the emission material powderinto a flowing gas, and pump the gas radially out through the pores ofthe electrode. Application of emission material to the face and to theouter cylindrical surface is carefully avoided. This is necessary inorder to maintain the arc in a diffused mode within the cavity andprevent it from burning elsewhere in a spot mode.

The great weight of the electrode requires that is be supported throughthe terminal member 7 and rod conductor 4 directly by means of someexternal fixture, and not through the vitreous envelope and the hermeticseal thereto. At the same time this arrangement permits dissipation ofelectrode heat through rod conductor 4 with the final result that thefront end of the electrode is much hotter than the shank end, forinstance 1690 C. vs. l040 C. a ratio of cavity depth to diameter in therange of lzlto 4:1 is generally desirable. The difference in temperaturebetween the front and shank end of the electrode, plus the fact thatcathode emitter coil 19 extends only a certain fraction (e.g'.three-fourths) of the cavity depth, assure that the arc burns toward thefront portion of the cavity. As arc current builds up during the cycle,heated gas tends to blow in and out of the dead space at the rear.However the gas in the positive column of the lamp heats upsimultaneously and there is less gas transfer back and forth through theelectrode opening than there is between live and dead spaces of thecavity. Effectively the positive column within the lamp causes expansionto the region back of the electrodes and to the walls as well. The deadspace within the lamp com prehends all such regions into which expansionoccurs. The dead space within the electrode cavity .is comparable to thedead space within the lamp. To a first approximation, when the ratio ofdead space to live space within the electrode is the same as the ratioof dead space to live space within the lamp volume, there is no motionof gas across the cavity mouth during the AC cycle, and blackening fromthis cause is minimized.

By way of example of the invention, in a 100 kw. lamp design, electrodesembodying the invention were constructed capable of handling 400amperes. The length of electrode body was 2 13/16 inch; outside diameter1 5/16 inch, cavity diameter 7/8 inch. The shank or rod conductor 4 was3/8 inch diameter tungsten rod. The cathode emitter coil 19 consisted of50 mil tungsten wire initially coated with a light (5 milligram) coatingof BaTh0 The quantity of BaTh0 embedded in the porous tungsten of theinterior wall was about 165 milligrams per electrode. The electrodeswere sealed into a quartz tube of 1.4 inch (3.5 cm.) internal diameter,as illustrated in FIG. 1. The tube or envelope was filled withsufficient xenon to provide an operating pressure from [78 to l V;atmospheres at a current intensity of 400 amperes. The lamp was providedwith a water-coolingjacket as illustrated in the drawing.

Tests of such lamps at 400 amperes show a wall-stabilized discharge anda rising volt ampere characteristic is exhibited, as shown by curve 2|in H6. 2. This means that such lamps can be operated without ballast orwith minimum ballasting provided for safety reasons only. The r.m.s.voltage drop in these lamps was about 2.5 volts per centimeter. Underthese conditions a 400 ampere lamp requires a length of about 1 meterfor a kilowatt input. The efficiency of such a lamp is about 36 lumensper watt.

1 claim:

l. A high current thermionic self-heating electrode comprising anelongated hollow tubular body of porous tungsten open at the front endand closed at the rear end. a coil of tungsten wire located within saidbody and lining the inside wall thereof, and extending less than thefull depth of said cavity, low work function emission material embeddedin the pores of the inside wall only of said electrode at leastcoextensively with said coil, and a heavy rodlike conductor supportingsaid electrode from the rear end and assuring a substantial temperaturedifference between the front end and the rear end thereof.

2. An electrode as defined in claim I wherein the emission material isan alkaline earth metal compound.

3. A high current (wall-stabilized) discharge lamp comprising a tubularvitreous envelope containing an ionizable filling and having a pair ofthermionic self-heating electrodes sealed into opposite ends, eachelectrode comprising an elongated hollow tubular body of poroustungstenopen at the front end in the direction of the arc and closed atthe rear end, a coil of tungsten wire located within said body andlining the inside walls only thereof and extending less than the fulldepth of said cavity, a low work function emission material embedded inthe pores of the inside wall only of said electrode at leastcoextensively with said coil, and a heavy rodlike conductor supportingsaid electrode from the rear end and assuring a substantial temperaturedifference between the front and the rear end thereof.

4. A lamp as defined in claim 3 wherein said coil of tungsten wireextends only to a depth within said cavity corresponding substantiallyto the live space thereof, the ratio of live to dead space within saidcavity corresponding to the ratio of live space within said lampoccupied by the discharge to dead space not so occupied.

5. A lamp as defined in claim 3 wherein the emission material is analkaline earth metal compound and the ionizable filling is an inert raregas.

6. A lamp as defined in claim 4 wherein the emission material is analkaline earth metal compound and the ionizable filling is an inert raregas.

7. A lamp as defined in claim 4 wherein the emission material is bariumthorate and the ionizable filling is xenon.

1. A high current thermionic self-heating electrode comprising anelongated hollow tubular body of porous tungsten open at the front endand closed at the rear end, a coil of tungsten wire located within saidbody and lining the inside wall thereof, and extending less than thefull depth of said cavity, low work function emission material embeddedin the pores of the inside wall only of said electrode at leastcoextensively with said coil, and a heavy rodlike conductor supportingsaid electrode from the rear end and assuring a substantial temperaturedifference between the front end and the rear end thereof.
 2. Anelectrode as defined in claim 1 wherein the emission material is analkaline earth metal compound.
 3. A high current (wall-stabilized)discharge lamp comprising a tubular vitreous envelope containing anionizable filling and having a pair of thermionic self-heatingelectrodes sealed into opposite ends, each electrode comprising anelongated hollow tubular body of porous tungsten open at the front endin the direction of the arc and closed at the rear end, a coil oftungsten wire located within said body and lining the inside walls onlythereof and extending less than the full depth of said cavity, a lowwork function emission material embedded in the pores of the inside wallonly of said electrode at least coextensively with said coil, and aheavy rodlike conductor supporting said electrode from the rear end andassuring a substantial temperature difference between the front and therear end thereof.
 4. A lamp as defined in claim 3 wherein said coil oftungsten wire extends only to a depth within said cavity correspondingsubstantially to the live space thereof, the ratio of live to dead spacewithin said cavitY corresponding to the ratio of live space within saidlamp occupied by the discharge to dead space not so occupied.
 5. A lampas defined in claim 3 wherein the emission material is an alkaline earthmetal compound and the ionizable filling is an inert rare gas.
 6. A lampas defined in claim 4 wherein the emission material is an alkaline earthmetal compound and the ionizable filling is an inert rare gas.
 7. A lampas defined in claim 4 wherein the emission material is barium thorateand the ionizable filling is xenon.